The invention relates to a method for error correction, in which mechanisms for repetition of data packets with errors are not possible owing to the excessive transmission delays associated with them.
During packet-oriented transmission of data, bit errors generally occur in landline networks owing to overloading at network nodes and, in the case of mobile channels, as a result of atmospheric interference and multipath propagation, and these bit errors are detected by error-recognition codes and lead to rejection of entire packets. So-called reliable transport protocols such as TCP use repetition mechanisms (Automatic Repeat Request Methods) in order to make it possible to transport packets which have been lost or rejected reliably to the receiver despite channels being subject to interference. However, this is associated with a very considerable increase in the transmission delay, which may however be intolerable, especially for communications purposes. Unreliable transport protocols, for example UDP, are therefore used in this case. Although these do not result in any increase in the transmission delay, they do not, however, offer any mechanisms for compensating for packet losses, either. For some time, the audiovisual transport group (AVT) of the IETF have been making intensive efforts to link methods for forward error correction (FEC) and packet-oriented transmission to unreliable transport protocols. In this case, by way of example, a plurality of data packets are coded in one step and the z code words cwi which result from this, where 0≦i<z, of length N are arranged in a memory array such that the code words form the rows or columns of a z×N or N×z matrix. Columns and/or rows are then mapped onto packets of a transport layer, for example RTP/UDP, and are transmitted. In order to make it possible to achieve source-adaptive coding, the code words cwi in a matrix may have different protection against errors and deletions.
If the expression epi is used in the following text to denote the correction capability of a code word cwi, then a so-called error protection profile EPP describes the correction capability of the set of code words which are combined in a matrix EPP={ep0, ep1, . . . epz−1}. The error protection profile for two successive matrices or transmission blocks may be different, depending on whether it relates to the matching of error protection to the source or else to the channel. However, in order to make it possible to decode the code words in a transmission block at the receiver, the corresponding error protection profile must be known to the decoder or must be transmitted to the decoder. This is dependent on reliable signaling, which should be efficient in order to keep the additional overhead as low as possible. It would be very inefficient to transmit a sufficiently large number of fields for this purpose in the header of each transmission packet, and transmission outside the logical channel (out-of-band signaling) involves additional complexity.